Just a few moments ago a huge list of what devices are expected to get an Ice Cream Sandwich update from Sony was released here on Babbling Boolean, now some of those select devices already have root methods. The first two devices are the Xperia Neo and Neo V which you can find the original instructions here.
For those of you with an updated Xperia Pro, Arc, Arc S you can follow the original instructions here, or a slightly easier alternated instructions here. Remember by attempting to root your device, you could permanently damage it, so you’ll be doing so at your own risk. Will you be rooting your device?